Within the semiconductor industry, an ever present need exists for improved process repeatability and control. For example, during the formation of a typical metal-layer-to-metal-layer interconnect, a dielectric layer is deposited over a first metal layer, a via hole is etched in the dielectric layer to expose the first metal layer, the via hole is filled with a metal plug and a second metal layer is deposited over the metal plug (e.g., forming an interconnect between the first and the second metal layers). To ensure the interconnect has low contact resistance, all dielectric material within the via hole must be etched from the top surface of the first metal layer prior to formation of the metal plug thereon; otherwise, residual high-resistivity dielectric material within the via hole significantly degrades the contact resistance of the interconnect. Similar process control is required during the etching of metal layers (e.g., Al, Cu, Pt, etc.), polysilicon layers and the like.
Conventional monitoring techniques provide only a rough estimate of when a material layer has been completely etched (i.e., endpoint). Accordingly, to accommodate varying thicknesses of material layers (e.g., device variations) or varying etch rates of material layers (e.g., process/process chamber variations), an etch process may be continued for a time greater than a predicted time for etching the material layer (i.e., for an over-etch time). Etching for an over-etch time ensures that all material to be removed is removed despite device variations and process/chamber variations that can vary etch time.
While over-etch times ensure complete etching, over-etching increases the time required to process each semiconductor wafer and thus decreases wafer throughput. Further, the drive for higher performance integrated circuits requires each generation of semiconductor devices to have finer dimensional tolerances, rendering-over-etching increasingly undesirable. The smaller open areas required for reduced dimension device structures also reduce the intensity of commonly monitored electromagnetic emissions (e.g., reaction product emissions) so as to render monitoring techniques employing narrow band intensity measurements increasingly difficult and inaccurate. Accordingly, a need exists for improved techniques for monitoring semiconductor manufacturing processes such as etch processes, chamber cleaning processes, deposition processes and the like.